A method and a decoder for determining erasures in an FEC (Forward Error Correction) decoding process decoding data encoded with concatenated codes is provided. First output data are generated by decoding first input data Second output data are generated by decoding second input data, the second input data including at least a part of the first output data. The first and the second output data are compared for updating a comparison result accumulation parameter based on the comparison result. Finally, it is determined whether an erasure is to be set based on the updated comparison result accumulation parameter.